Displays are a critical technology for the nation; they are the primary means by which people receive information from machines, ranging from computer screens to hospital monitors to head-mounted displays. Increasingly, flat panel displays (FPDs) are becoming more important for meeting military and civilian application requirements. Such FPDs are, both literally and figuratively, windows into today's information age. However, current display technologies are limited by their range of sizes, power consumption, lifetime, robustness, and operational conditions. A variety of different size FPDs are urgently needed that are lightweight and economical while having lower power consumption and higher resolutions. Future information systems, both military and civilian, will become increasingly constrained by the availability and performance of their display interfaces.
Two major problems plague phosphors in use in display technologies. The first is the inability to produce phosphor materials with sufficient stability and longevity to satisfy the lifetime requirements of a display. The second problem is the inability to prepare phosphors that emit red, green and blue light with sufficient brightness to satisfy luminance requirements for ease of viewing.
The development of thin film phosphors has been seen as one solution to the current limitations of existing phosphors. Phosphor materials in thin film form offer several distinct advantages over the conventional powder phosphor screens. Fully dense phosphor films have less surface area and outgas less than powder phosphors. Typically, thin film phosphors have smaller grain sizes than conventional powder phosphor materials. The smaller grain size provides a smaller spot size and correspondingly higher resolution. Because the film is fully dense and in intimate contact with the underlying substrate, thin film phosphors transfer heat more effectively than conventional porous powder phosphors. This allows the thin film device to be driven at higher power levels, and therefore produce higher luminance. Thin film phosphors can also rely on the experience of the microelectronics industry for the efficient scale up of the deposition processes to commercial levels.
Further improvements in the phosphor brightness, chromaticity, efficiency, and turn-on voltage have been sought through improvement of the deposition process for the phosphor materials, especially by deposition processes achieving low temperature growth of crystalline-as-deposited materials. Previously, deposition techniques for thin film phosphors relied on physical vapor deposition techniques such as sputtering, multi-source evaporation, or molecular beam epitaxy. These techniques are typically slow and produce amorphous films requiring an extra step of a post-deposition anneal. Also, such depositions can suffer from large defect densities because of the volatility of some of the constituents or because of thermal instability of the phosphor.
The development of low temperature metal-organic chemical vapor deposition (MOCVD) for the growth of thin film phosphors was a significant advancement in the area of thin film phosphors. The low temperature (&lt;600.degree. C.) MOCVD of crystalline-as-deposited alkaline earth thin film phosphors (cerium-doped calcium thiogallate) has recently been demonstrated. This material showed photoluminescence, cathodoluminescence, and electroluminescence. An electroluminescence brightness of over 7.5 cd/m.sup.2 at a drive frequency of 60 Hz with approximately a 40 V reduction in the threshold voltage has been observed for the MOCVD CaGa.sub.2 S.sub.4 :Ce approaching the highest reported value for sputtered CaGa.sub.2 S.sub.4 :Ce of 10 cd/m.sup.2. However, the sputter deposited materials require a thermal anneal after deposition at temperatures in excess of 650.degree. C. This extra annealing step limits the usable substrate glasses to high temperature varieties. The extra step and the necessity of high temperature glasses add significantly to the cost of any subsequent device.
Further improvement of Ce-activated alkaline earth thiogallates is desired for EL devices to become fully utilized. Present state of the art full color EL displays use sputtered CaGa.sub.2 S.sub.4 :Ce as the blue phosphor. This material has a luminance of 10 cd/m.sup.2, compared to 54 cd/m.sup.2 for the red (ZnS:Mn with a color filter) and to 43 cd/m.sup.2 for the green (ZnS:Tb). As a result of the relative dimness of the blue material, a higher fill factor is needed to provide a brightness that is agreeable to the human eye. In one present EL device, the CaGa.sub.2 S.sub.4 :Ce has a fill factor of 48%, meaning that almost half of the full color pixel is composed of the blue phosphor. Thus, the ultimate resolution of the display is limited by the brightness of the blue phosphor because it defines the amount of material necessary to make up the pixel. Further, from a manufacturing prospective, the use of large amounts of CaGa.sub.2 S.sub.4 :Ce is not desirable because of the high cost from the expensive starting materials, extra processing steps, and the high temperature glass.
It is an object of this invention to provide an increase in brightness in blue phosphor emission.
It is a further object of this invention to provide an increase in brightness in blue phosphor emission without effecting the chromaticity of the blue color emission.
Yet a further object of this invention is to provide an improved substrate for subsequent MOCVD deposition of a phosphor especially of a blue phosphor such as CaGa.sub.2 S.sub.4 :Ce.
A still further object of the present invention is to allow for the MOCVD deposition of the required phosphors with increase brightness but without the need for a subsequent high temperature anneal requiring the use of a special high temperature glass.